You announce the value of X as you activate the ability, and all instances of X in the activation cost are equal to the announced value. For example, if you choose 2 as the value of X, then you pay and remove two +1/+1 counters to pay the cost.

2/1/2014

The amount of mana you spent to cast this creature is usually equal to its converted mana cost. However, you also include any additional costs you pay, including the cost imposed for casting your commander from the command zone.

If Marath enters the battlefield without being cast, then no mana was spent to cast it. It will therefore enter the battlefield without any +1/+1 counters. If no other effects are increasing its toughness at that time, it will subsequently be put into its owner’s graveyard as a state-based action.